Study on six-photon and five-photon ultraviolet upconversion luminescence

The ultraviolet upconversion luminescence of the Tm3+ ion sensitized by the Yb3+ ion in fluoride glass as excited by a 975 nm diode laser was studied. Two typical ultraviolet upconversion luminescence lines positioned at 290.6 and 362.0 nm were found and could be identified as the fluorescence transitions of 1I6→3H6 and 1D2→3H6, respectively. The elaborative measurement of the variation of upconversion luminescence intensity F as a function of the 975 nm pumping laser power P has proven that the luminescence lines result from five-photon and six-photon upconversion processes, respectively. To our best knowledge, the finding of six-photon ultraviolet upconversion luminescence is original. Moreover, a novel characteristic upconversion luminescence saturation phenomenon was observed, as the logF-logP curve of upconversion fluorescence versus pumping laser power is a straight line, while the slopes of these logF-logP plots are apparently smaller than the value determined by the normal multiphoton relation. When the pumping laser spot was increased, the slopes were enhanced noticeably from small values to those that are equal to or near what a normal multiphoton relation would determine. The mechanism of this characteristic saturation phenomenon is the energy expansion resulting from the energy resonant migration among Yb3+ ions.